Click here for table of contentsClick here to searchFREE-STANDING MASONRY PRIVACY WALLSRod JohnstonPrincipal of Electronic BlueprintPrincipal of Quasar Management Services Pty Ltd(Consultant to the Concrete Masonry Association of Australia)SUMMARYFree-standing masonry privacy walls must be designed and constructed to withstand a rangeof loads, and in particular, wind and earthquake loads. This paper provides a detaileddescription of the design process and the determination of: Wind loads for various locations and exposures Earthquake loads Active and passive soil pressures that affect the stability of the system Pier dimensions to provide stability, including the relevant structure/soil interaction Pier and masonry reinforcement design Detailing of masonry privacy walls.BACKGROUNDThe collapse of a number of free-standing masonry privacy walls under extreme wind hasprompted the Queensland government consider regulating their design and construction. Freestanding masonry privacy walls must be designed and constructed to withstand a range ofloads, and in particular, wind loads. There are several possible designs for masonry privacywalls, two of which are shown below. The diagrams and tables herein are from ConcreteMasonry Fences, Data Sheet 5, Concrete Masonry Association of Australia, May 2007.
Click here for table of contentsClick here to searchREINFORCED MASONRY WALL ON CONCRETE STRIP FOOTINGSThe type of retaining wall shown in Figure 1 may be designed using the principles set outbelow for a wall with reinforced concrete piers (as per Figure 2), except that the resistance tooverturning is provided by the combined weight of the wall and footing acting about anassumed point of rotation close to the toe of the footing. The distance from the toe to the pointof rotation depends on the bearing capacity of the foundation soil, including its compaction. Itthe soil is firm with a high bearing capacity, the point of rotation will be close to the toe. If thesoil is soft with a low bearing capacity, the point of rotation will move closer to the centre ofthe footing. A reasonably conservative assumption is that the point about which the footingrotates is approximately B/3 from the toe of the footing, where B is the total footing width.REINFORCED MASONRY WALL WITH REINFORCED CONCRETE PIERSIn most circumstances, the economical form of construction for free-standing masonryprivacy walls is as shown in Figure 2. The wall consists of 190 mm hollow concreteblockwork, with a reinforced bond beam and capping block at the top and a reinforced bondbeam at the bottom. The bond beams should include a single horizontal 16 mm diameterreinforcing bar, set in 190 mm knock-out bond beam blocks. The wall is supported, at centresranging from 1.8 m to 3.0 m, by 450 mm diameter reinforced concrete piers, constructed inholes bored to the required depths and spacings. The depths of 450 mm piers, for variouscombinations of pier spacing, soil type (internal friction angle), wall heights and windclassifications, may be calculated by the methodology shown below. Each pier should includeone (or more) reinforcing bar, which extends up and is grouted into the 190 mm concreteblockwork. The required number of vertical bars depends on the spacing of the piers, the wallheight and wind classification.WIND LOADSWind loads on free-standing masonry privacy walls should be calculated using AS/NZS1170.2. However, designers often associate these structures with the design of houses to AS4055. Strictly speaking, masonry privacy walls are outside the scope of AS 4055, although thenomenclature used therein is useful in classifying the wind exposure of housing sites for windloads on such structures.
Click here for table of contentsClick here to searchEARTHQUAKE LOADSEarthquake loads should be calculated using AS 1170.4. Method EDC I. For simple structuresin most Australian applications, this permits the lateral earthquake inertia load to be assumedto be 10% of the seismic weight. For a typical 190 mm hollow block privacy wall, the averageblockwork weight is 245 kg/m2 or 2.40 kN/m2, and the resulting horizontal earthquake inertiaforce is 0.24 kPa. This is significantly less that the expected wind loads shown above.SOIL PROPERTIESSoil properties used to determine the resistance to overturning of the piers should be based onreduction factors given in AS 4678 and “cautious estimates of the mean” density, internal andexternal friction angles and cohesion.PIER RESISTANCEThe overturning resistance of piers supporting free-standing masonry privacy walls may bebased on the principles for laterally loaded “short” piles set out in AS 2159.The method of determining the soil lateral resistance, employed in the worked examplebelow, is based on Lateral Resistance on Piles in Cohesionless Soils, by B.B. Broms (May1964). For a single short pier in cohesionless soil, this paper suggests that the resistance bedetermined from the design passive resistance multiplied by a factor of 3.0 (designated kpier inthe example). A similar paper, Lateral Resistance on Piles in Cohesive Soils, by B.B. Broms(March 1964) covers cohesive soils. There are other more recent papers describing methodsof varying complexity, based on tests and/or theory. However, the selected method has beenchosen for its simplicity, in the context of the fact that there is relatively low risk associatedwith privacy walls with a maximum height of approximately 1.8 m.The assumed distribution of pressures resisting the overturning moments are as follows.
Click here for table of contentsClick here to searchWORKED EXAMPLESet out in the following pages is a worked example, the purpose of which is to demonstratethe method by which free-standing masonry privacy walls may be designed for a particularwind and earthquake loads, and soil type. It also may serve as a test for any softwaredeveloped for designing masonry privacy walls.Design BriefThe objective is to design a 1.8 m high free-standing masonry privacy wall located in aSydney suburb, on a gentle slope (with 60 metres upwind distance to the crest of a 4.0 m hill)and shielded by houses of 3.0 m roof height and 7.0 m width. The piers will be set in “in-situ”sandy-clay material (with cautious estimates of the means of density 20 kN/m3, internal angleof friction 30o and cohesion 5.0 kPa).Wind Load using AS/NZS 1170.2-2002RegionADegree of hazard2LocationNon-cyclonicDesign event for safety1 in 500Regional wind speed,VR 45 m/sAS/NZS 1170.2 Table 3.1AS/NZS 1170.2 Clause 3.3.1Regional wind multiplier,Md 1.0Terrain category multiplier, h 3.0 m Mz, cat 0.91AS/NZS 1170.2 Table 4.1(A)oNumber of upwind shielding buildings within a 45 sector of 20 h radius, ns 2Average roof height of shielding,hs 3.0 mAverage spacing of shielding buildings, ls h (10 / ns 5) 1.8 ([10 / 2] 5) 18.0 mAverage breadth of shielding buildings, bs 7.0 mShielding parameter,s ls / (hs bs)0.5 18.0 / (3.0 x 7.0)0.5 3.93 AS/NZS 1170.2 Table 4.3.3Shielding multiplier,Ms 0.830 Interpolated AS/NZS 1170.2 Table 4.3Height of the hill, ridge or escarpment, H 4.0 mHorizontal distance upwind from crest Lu 60.0 mWindward slopeH/2Lu 4.0 / (2 x 60.0) 0.033 0.05AS/NZS 1170.2 Clause 4.4.2Topography multiplierMt 1.00Ultimate design gust wind speed,Vz u VR Md (Mz, cat Ms Mt) 45.0 x 1.0 x 0.91 x 0.830 x 1.0 34.0 m/sUlt free stream gust dynamic pressure, qzu 0.0006 Vz u 2AS/NZS 1170.2 2.4.1 0.0006 x 34.02 0.69 kPaStructure GeometryHeight of wall,h 1.8 mSolid height of wall,c 1.8 mTotal length of wall,b 9.0 mLength/solid height,b/c 9.0 / 1.8 5.0Solid height/total height,c/h 1.8 / 1.8
Click here for table of contentsClick here to search 1.0θ 0Kp 1 – (1 – δ)2 1 – (1 – 1)2 1.0Length of wall between vertical supports, B’ 2.4 mAngle of incident wind (Normal 0),Porosity reduction factor,Wind LoadNet pressure coefficient,Aerodynamic shape factor,Ultimate net wind pressure,Earthquake LoadHazard factor for SydneySubsoil classificationSeismic weightLateral earthquake loadAS/NZS 1170.2 D2.1Cpn 1.3 0.5 (0.3 log10 (b/c]) (0.8 – c/h) 1.3 0.5 (0.3 log10 (5.0]) (0.8 – 1.0) 1.20AS/NZS 1170.2 Table D2(A)Cfig Cpn Kp 1.20 x 1.0 1.20AS/NZS 1170.2 D1.4pnu Cfig qzu 1.20 x 0.695 0.834 kPaAS/NZS 1170.2 2.4.1z 0.08CW 2.4 kN/m2pe 0.10 W 0.10 x 2.40 0.240 kPa 0.834 kPa Base design on windLoad factors and capacity reduction factorsLoad factor on overturning windGw 1.0Load factor on restoring forcesGr 0.8Shear force and bending moments at the base of wallShear force at baseVb γ pn u B’ h 1.0 x 0.834 x 2.4 x 1.80 3.60 kNBending moment at baseMb 0.5 Gw pn u B’ h2 0.5 x 1.0 x 0.834 x 2.4 x 1.80 2 3.22 kN.mFoundation soilThe piers will be set in “in-situ” sandy-clay material with the following properties.Any over-excavation should be filled with compacted cement-stabilised road base.Design will be to the principles set out in AS 4678.Densityρf 19.6 kN/m3 (Cautious estimate of mean)(Cautious estimate of mean)Internal angle of frictionφf 30 o(Cautious estimate of mean)Cohesion,cf 5.0 kPaDesign properties of soilФ tan(φf) 0.85Foundation soil factor on tan(φf)Foundation soil factor on cohesionФcf 0.70Foundation soil design internal friction φ*f tan-1 [Ф tan(φf). tan(φf)] tan-1 [0.85. tan(30o)] 26.1o
Click here for table of contentsFoundation soil design cohesion,Passive pressure coefficientClick here to searchc*f Фcf . cf 0.70 x 5.0 3.5 kPaKp 1 sin(φ*f)1 - sin(φ*f) 1 sin(26.1o)1 - sin(26.1o) 2.58Pier detailsTotal depth of pier,D 0.900 m This value will be checkedPier diameter,dpier 0.450 mThe following calculations convert a circular pier to an equivalent square pier of thesame overall cross-sectional area. By using this effective square section, the designercan have confidence in the calculated weight of pier, and the effective horizontal leverarms from an assumed point of rotation.Effective pier thickness,Tp (3.1416 / 4)0.5 dpier (3.1416 / 4)0.5 0.450 0.399 mEffective pier length along the wallLp 0.399 mOverturning AnalysisWhen piers push into a soil, the resistance is significantly greater than the passive resistancenormally associated with the cross section of the pier. The multiplier to account for thisincreased lateral resistance of piers pushing into a body of soil is assumed to be kpier 3.0As the horizontal force increases, the wall support will rotate about its base, pushing forwardinto the soil. The movement will vary linearly from the maximum at the ground surface tozero at the bottom of the support.The resistance to this movement is provided by the passive resistance of the soil in front ofthe support. Under uniform movement, the passive pressure varies uniformly from zero at thesurface to a maximum at the base of the support.Passive force over the total depth,Pp Gr kpier Kp ρ Lp D2 / 3 0.8 x 3.0 x 2.58 x 19.6 x 0.399 x 0.9002 / 3 13.1 kN.mLever arm of passive force,yp D / 2 0.900 / 2 0.450 mRestoring moment due to passive force Mp Pp yp 13.1 x 0.450 5.87 kN.mFactored weight of wall ,Pvw Gr ρw h t b 0.8 x 16.0 x 1.800 x 0.19 x 2.4 10.5 kNLever arm of wall weight,yw Tp (0.5 – 0.167) 0.399 (0.5 -0.167) 0.133 mRestoring moment due to wall weight, Mw Pvw yw 10.5 x 0.133 1.40 kN.m
Click here for table of contentsFactored weight of pier/footing,Lever arm of pierRestoring moment due to pierTotal restoring moment,Bending moment due to windClick here to searchPvf Gr ρf Tf Lf D 0.8 x 23.5 x 0.399 x 0.399 x 0.900 2.69 kNyf Tp (0.5 – 0.167) 0.399 (0.5 -0.167) 0.133 mMf Pvf yf 2.69 x 0.133 0.36 kN.mMR Mp Mw Mf 5.87 1.40 0.36 7.63 kN.mMb Gw pn u B h (h/2 D) 1.0 x 0.834 x 2.40 x 1.80 x (1.80 / 2 0.900) 6.48 kN.m 7.63 kN.m OKReinforced Masonry Pier Design Concrete blocks:Width 190 mm, Strength grade 15 MPa Blockwork will be built continuous for a length of 2.4 m, with a pier located at thecentre and articulation joints (joints permitting relative movement due to soilexpansion and contraction) at each end. Main reinforcement 1-16 mm diameter bar in the centre of the pier (500 MPa yield)Masonry PropertiesBlock characteristic compressive strength f'uc 15.0 MPaBlock type factor,km 1.6 (Hollow blocks)Equivalent brickwork strength,Mortar joint height,Masonry unit height,Ratio of block to joint thickness,Block height factorCharacteristic masonry strength,f'mb km (f'uc)0.50.5 1.6 (15.0) 6.20 MPahj 10 mmhb 190 mmhb/hj 190/10 19.0kh 1.3f'm kh f'mb 1.3 x 6.20 8.06 MPaConcrete Grout PropertiesConcrete grout shall comply with AS 3700 and have: a minimum portland cement content of 300 kg/cubic metre; a maximum aggregate size of 10 mm; sufficient slump to completely fill the cores; and a minimum compressive cylinder strength of 20 MPa.Specified characteristic grout strengthf’c 20 MPa 12 MPa OK AS 3700 Clause 5.6Design characteristic grout strength,f’cg min [(1.3 x f'uc), 20.0] AS 3700 Clause 3.5 min [(1.3 x 15), 20.0] min [19.5, 20.0] 19.5 MPa
Click here for table of contentsClick here to searchDimensions and Properties of Reinforced Concrete Masonry PierThe most adverse loading is on the pier near the middle of the wallWidth of pier (along the wall),B 390 mmDepth of pier (through the wall),D 190 mmDensity of reinforced concrete masonry,ρ mas 2,200 kg/m3Modulus of elasticity,E 1,000 f’m 1,000 x 8.06 8,060 MPaSecond moment of area,I B D3 / 12 390 x 1903 / 12 222.9 x 106 mm4Main ReinforcementMain reinforcement yield strength,Main reinforcement shear strength (dowel),Number of main tensile reinforcing bars,Diameter of main tensile reinforcing bars,Area of main reinforcement,Effective depth of reinforcement,Effective width of reinforced section,Shear width of reinforced section,Design area of main tensile reinforcement,fsy 500 MPafsv 17.5 MPaNt 1Ddia t 16 mmAst 200 mm2d D/2 (Centrally located reinforcement) 190/2 95 mmb